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Quarks break free at two trillion degrees

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The latest version of the QCD phase diagram. The boundary between the normal (hadronic) low-temperature phase and the high-temperature quark–gluon plasma phase is marked in black. The square box on the solid line indicates the yet-to-be-found critical point where phases can co-exist. Neutrons and protons and other ordinary matter particles (including antimatter particles) are detected after they "freeze out" of fireballs caused by heavy-ion collisions like those at RHIC, indicated by the dotted line. To the right is a possible region of "colour superconductivity". (Courtesy: Gupta et al.)

Physicists in the US, India and China have calculated that quarks and gluons can break free from their confinement inside protons and neutrons at a temperature of around two trillion degrees Kelvin – the temperature of the universe a fraction of a second after the Big Bang. The researchers arrived at this figure by combining the results of supercomputer calculations and heavy-ion collision experiments. They say that it puts our knowledge of quark matter on a firmer footing.

According to the Big Bang model, the very early universe was filled with “quark–gluon plasma”, in which quarks and gluons (the carriers of the strong nuclear force) existed as individual entities. The strong force between quarks increases rapidly with distance, which means that the quarks need large amounts of energy to remain free – and therefore the plasma can only exist at extremely high temperatures. When the cosmos was only about a millionth of a second old, it had cooled to the point where quarks and gluons combined to form composite particles such as protons and neutrons. Exactly what this temperature is, however, has not been easy to work out.

The theory of quantum chromodynamics (QCD) explains….. Read the rest of this entry »

Written by physicsgg

June 23, 2011 at 8:15 pm

Posted in High Energy Physics

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Quick Reminder on Top Quarks

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“….Top quark pairs may be produced at the Tevatron proton-antiproton collisions when a energetic quark and its antiparticle hit each other head on; this happens only if the total energy of the collision is larger than twice the mass of the top quark. Since the top is very massive (172 GeV or so), such collisions are very rare, with the result that only once in ten billion proton-antiproton collisions a top quark pair pops into existence.
Being very massive, the top quark is very eager to disintegrate. It does so at a fantastic speed, turning into a W boson and a b-quark. The b-quark produces a hadronic jet -a stream of collimated, energetic hadrons; the W boson decays into an electron-neutrino pair, or a muon-neutrino pair, or a tauon-neutrino pair, or a pair of quarks. Since quarks come in three colours and there are two possible quark pairs into which the W can decay (ud or cs, while quarks of the third family are prevented by the too large mass of the top), there are a total of six possible quark pair decays against only three leptonic decays, so the democratic W boson chooses to go one ninth of the time into each.
The final result of the top pair production and decay chain thus involves two b-jets (one from each bottom quark produced when each top emits a W), and either four additional hadronic jets (from two W->qq decays); or two jets and a lepton-neutrino pair; or two lepton-neutrino pairs. Physicists call these signatures “all-hadronic”, “lepton plus jets”, and “dileptonic”, because the distinguishing feature is the presence of zero, one, or two charged leptons.
Note that the all-hadronic signature arises 6/9 * 6/9 = 44% of the time; also note that by “leptons” experimentalists oftentimes mean electrons or muons, since tau leptons are harder to identify with certainty. So the “single lepton” final state arises 2 * 6/9 * 2/9 of the time (the first factor of two is due to the fact that either the first or the second W can go into quarks), or 30%. The “dilepton” topology is just happening 2/9 * 2/9 of the time, or 5%.
The above classification has been a standard ever since top quarks were shown (in 1988) to have a mass larger than the W boson, such that the t->Wb decay was then practically the only one occurring with a significant rate. The first observation of top quarks in 1995 was produced  in samples of single lepton and dilepton candidates: these are the ones with the best signal to background ratio. Then, in 1997 a difficult fight with the much higher background from strong interaction processes (which readily produce a six-jets final state at a rate that totally drown the tiny top signal) allowed my colleagues in Padova and I to first observe the all-hadronic decay of top quark pairs….”

http://www.science20.com/quantum_diaries_survivor/top_cross_section_measurement_i-78971

Written by physicsgg

May 14, 2011 at 4:07 pm

Posted in High Energy Physics

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